Variable capacitors are used in many applications, such as matching networks and variable filters. They allow for the precise tuning, after assembly, of frequency and/or impedance in applications needing a dynamic system response, such as in plasma processes. The ability to dynamically change impedance and frequency response provides more flexibility for the applications variable capacitors are used in, and can compensate for variations from unit-to-unit. Some examples of variable capacitors are vacuum variable capacitors (VVCs) and electronically variable capacitors (EVCs).
In electronic circuits, matching networks are used to match the source impedance to the load impedance and vice versa. That is, the source, being of some impedance with a resistive part and a reactive part, will be terminated into the complex conjugate impedance, and the load impedance will be driven by the complex conjugate of its impedance. The complex conjugate is used to eliminate the reactive part of the impedance, leaving only the resistive part, and the resistive part is made equal. This is done so that maximum power transfer can be achieved at the load.
In plasma applications, the load impedance can vary depending on several factors, such as time, power level, pressure, gas flow, chemistry of the gasses, and whether the plasma has been struck. Accordingly, the matching network must be able to automatically vary itself to ensure that the maximum power transfer is achieved. This helps with repeatability in both the depositing and etching.
EVCs use switches to add or remove fixed capacitors, such as an MLCC (multi-layer ceramic capacitor), in a circuit. The capacitor and switch are placed in series. This circuit is then placed in parallel with other capacitor/switch circuits. The parallel circuits allow the capacitors to be simply added or subtracted in the circuit, depending on how many switches are opened or closed. In the case where all the switches are open, the EVC will be at its lowest capacitance value. In the case where they are all closed, the EVC will be at its highest capacitance value.
There are different approaches for arranging and choosing the capacitors of the EVC such that the EVC can provide progressively increasing capacitance values. There is need for an arrangement of capacitors for an EVC that provides the needed capacitance values without exceeding an individual capacitor's current rating. There is also need for an arrangement using a lower number of capacitors, switches, and associated hardware, and thus taking up less space.